here is the problem with bernie, (amongst other things) ... he may very well win the democratic nomination, but it might be something like 60-40 of dems who vote for him. of the 40% that don't vote for him, there WILL be people who don't vote for him in the main election either, or straight up vote for trump- whether that be because they find him too liberal, or they refuse to reward those who screwed Hillary and helped put trump in the white house to begin with, or because they just don't fall for the bs he shouts out like his mainly white millennial base does. On the flip side, if he doesn't win the nomination, there is the risk of his supporters not voting (again), in an attempt to prove some point that nobody else can see or understand. it sounds stupid, but people are stupid... and petty and opinionated... and with the republicans already rigging the elections, bernie just being in it at all creates the potential for an easy win (and a win that is difficult to dispute) on the republican's side... which is why trump and the gop wants him to be the nom.
and if Stacy Abrams goes as a vp it will probably be with Biden. She has already come to his defense several times, and I think she had a meeting with Obama at one point, who is pro Biden. But who knows, a lot of people are thinking it might be Kamala Harris that goes with Biden.